A semiconductor production equipment by thermally processing a wafer is required to control a temperature difference on the surface of the wafer to be minimized (e.g., within ±1° C.) in order to avoid problems such as thermal strain. Also, the equipment is required to rapidly increase a temperature (e.g., 100° C./sec) up to a desired temperature (e.g., 1350° C.). Then, an inductive heating device is widely known, in which an inductive heating coil is divided into multiple pieces and high frequency power supply (e.g., an inverter) is individually connected to each piece for performing power control. However, since the divided inductive heating coils are mutually adjacent, mutual inductances M are present to cause mutual inductive voltages. Therefore, respective inverters are operated in parallel with mutual inductances in between, and this causes mutual power exchanges between the inverters when there are phase shifts in electric currents of respective inverters. That is, since phase shifts are caused in magnetic fields among the divided inductive heating coils due to phase shifts in the electric currents in the respective inverters, magnetic fields in the vicinity of the boundary of the adjacent inductive heating coils are weakened to reduce the density of heat generated by inductive heating power. As a result, temperature variations may be caused on the surface of the heated object (such as a wafer).
Then, a technique of “ZONE Controlled Induction Heating” (ZCIH) has been proposed by the present inventor and others, in which even under conditions where mutual inductances are present to cause mutual inductive voltages between adjacent inductive heating coils, inductive heating power can be controlled as appropriate so as to prevent circulation currents from flowing between mutual inverters and to prevent heat generation density from decreasing in the vicinity of the boundary of the divided inductive heating coils (see Japanese Patent Application Publication No. 2011-014331 A (Claim 1, paragraph 0061 in Description) for example). According to this technique of ZCIH, each power supply unit is configured to individually include a step-down chopper and a voltage inverter (hereinafter, simply referred to as an inverter). Respective power supply units divided in multiple power supply zones are separately connected to respective inductive heating coils divided into pieces to supply power.
At this time, a current synchronization control (i.e., synchronization control of a current phase) is performed for an individual inverter in each power unit, that is, current phases flowing through respective inverters are synchronized to avoid circulation currents from flowing between multiple inverters. In other words, exchanging electric currents between multiple inverters are avoided to prevent overvoltage from being caused due to regenerative power flowing into inverters. In addition, by synchronizing current phases flowing through respective induction heating coils divided into pieces, with a current synchronization control of inverters, the heat generation density by the inductive heating power is prevented from sharply decreasing at the vicinity of the boundary of respective inductive heating coils. Further, by controlling input voltage of each inverter by way of each step-down chopper, a current amplitude control is performed for each inverter to control inductive heating power to be supplied to each inductive heating coil.
Japanese Patent Application Publication No. 2011-014331 A (Claim 1, paragraph 0061 in Description) describes a technique of performing a frequency sweep from a frequency higher than the resonant frequency downward, selecting a unit that first reaches a resonance point, and driving all the inverter circuits with the same switching frequency as that resonant frequency. This allows the technique described in Japanese Patent Application Publication No. 2011-014331 A (Claim 1, paragraph 0061 in Description) to maintain L-controlled driving at all the inverter circuits.